concentration gradient through specialized carrier proteins (illus-
trated on the lower surface of the thylakoid). These carrier proteins
are unusual because they function both as an ion channel and as an
enzyme. As hydrogen ions pass through the channel portion of the
protein, the protein catalyzes a reaction in which a phosphate group
is added to a molecule of ADP, making ATP. Thus, the movement of
hydrogen ions across the thylakoid membrane through these pro-
teins provides the energy needed to make ATP, which is used to
power the third stage of photosynthesis.
While one electron transport chain provides energy used to make
ATP, a second electron transport chain provides energy used to
make NADPH. is an electron carrier that provides the high-
energy electrons needed to make carbon-hydrogen bonds in the third
stage of photosynthesis. The second electron transport chain shown
in Figure 8 lies to the right of the second green pigment molecule. In
this second chain, excited electrons combine with hydrogen ions as
well as an electron acceptor called NADP+, forming NADPH.
The light-dependent reactions of photosynthesis can be summa-
rized as follows. Pigment molecules in the thylakoids of chloroplasts
absorb light energy. Electrons in the pigments are excited by light and
move through electron transport chains in thylakoid membranes.
These electrons are replaced by electrons from water molecules, which
are split by an enzyme. Oxygen atoms from water molecules combine
to form oxygen gas. Hydrogen ions accumulate inside thylakoids,
setting up a concentration gradient that provides the energy to make
ATP and NADPH.
NADPHSECTION 2 Photosynthesis 101
Identifying a Product
of Photosynthesis
You can use the following procedure to identify the
gas given off by a photosynthetic organism.
Materials
MBL or CBL system with appropriate software, test tube or
small glass jar, sprig of Elodea,distilled water, cool light
source, dissolved oxygen (DO) probeProcedure
1.Set up an MBL/CBL system
to collect and graph data
from a dissolved oxygen
probe at 30-second intervals
for 60 data points. Calibrate
the DO probe.- Place a sprig of Elodea
in a test tube or glass
jar, and fill the test tube or jar
with distilled water.
3.Place the test tube or glass
jar under a cool light source,
and lower a DO probe into
the water. Collect data for 30
minutes.
4.When data collection is
complete, view the graph of
your data. If possible, print
the graph. Otherwise, sketch
the graph on paper.Analysis- Infer the cause of any
change you observed. - Propose a control for this
experiment.
3.Critical Thinking
Evaluating Hypotheses
Explain how your data support
or do not support the hypoth-
esis that photosynthetic
organisms give off oxygen.
4B